Hitherto, instant lettering has been performed with an image-transferring material having printed thereon images such as letters, figures and indicia which are brought into contact with a surface of an image-receiving material and transferred thereto by applying pressure under dry condition. As a base sheet of such an image-transferring material (hereafter referred to as "dry-processing type image-transferring material"), transparent or semi-transparent plastic films or those having repelable coatings provided thereon are used. A conventional dry-processing type image-transferring material comprises a base sheet having thereon an image printed by screen-printing and further thereon a pressure-sensitive adhesive layer, with which the printed image is transferred to a image-receiving material which is brought into contact with the image-transferring material.
A base sheet for dry-processing type image-transferring materials which have an image printed by a thermally image-transferring process is described in JP-A-63 128987 (the term "JP-A" used means an unexamined and published Japanese patent application), wherein plastic films (e.g., a polyethylene film, a polypropylene film and a fluorine-containing resin film) having a smooth surface and exhibiting a contact angle with water of at least 95.degree., and paper, metal foils or plastic films having a coating of a silicone resin are used as the base sheet.
In order to thermally print an image onto a base sheet having a contact angle with water of at least 95.degree. and particularly not less than 105.degree. it is necessary to reduce surface tension of ink to wet the sheet and further necessary to increase adhesion between the ink and the sheet more than cohesive force of the ink and adhesion between the ink and a sheet of an ink-releasing sheet such as polyethylene terephthalate film. For the purpose, the ink temperature must be increased when the image is thermally printed, requiring high energy to be applied to a thermally image-transferring device, which is disadvantageous in view of durability of a thermal head and load on a power supply.
Further an image thermally printed on the base sheet having poor wetability is easily retransferred with slight pressure due to weak adhesion to the base sheet, and a portion of the image which is desired to be left on the base sheet may unwillingly be retransferred, causing stains on an image-receiving material. Such easy transfer is troublesome in handling of the sheet.
Furthermore, since the base sheet has an extremely small static friction coefficient, the sheet is not easily fixed during retransfer of the thermally printed image from the sheet to an image-receiving material, so that the image is retransferred on an undesired portion of the image-receiving material or distorted on the image-receiving material.
Japanese Patent Application No. 62-80127 discloses formation of a sticky layer apart from a thermally printed image on a base sheet so as to prevent the sheet from moving during retransfer of the image with pressure. However, the formation of a sticky layer on a certain portion of the base sheet necessitates a specific means in production and an exclusive device therefor, requiring large costs. Further, the sticky layer has to be covered with a separable sheet before use, i.e., before the image-retransfer step, which requires additional means and costs. Moreover, in the case of a base sheet having a repelable coating of a silicone resin, the base sheet is coated twice for the repelable coating and the sticky layer, and regardless of coating order of the two, the later coating may have chance to be affected by the previous coating.
That is, when a silicone resin is first coated on the sheet, the subsequent coating of a sticky composition is repelled. When the sticky composition is first coated, on the other hand, the silicone resin is coated only with difficulty because of stickiness of the previous coating. If the sticky layer is covered with a separable sheet, then the thickness of the resulting sheet partially increases so that the silicone resin cannot easily be coated. Even if the above processing works to properly coat the silicone resin, since the sticky layer is provided only at a certain portion of the sheet and not around images thermally printed on the sheet, the sheet still moves during the image-retransfer step resulting in formation of imperfect images on the image-receiving material.